In order to find the value of z, proceed as follow:
By using the Pythagorean theorem for the bigger triangle, you obtain:
Now, if you consider the second bigger triangle:
And in the smaller triangle:
Next, replace the second equation for y^2 into the previous one:
Now, replace the previous expression for x^2 into the first equation for z^2 and simplify:
Hence, the value of z is 2√22